In recent years, there has been proposed a technology in which communication of user data (data of User-Plane) is directly performed between a plurality of radio terminals without involvement of a radio base station (D2D communication). The user-data communication directly performed between the plurality of radio terminals, is performed by using a part of radio resources assigned to a mobile communication system. However, in the D2D communication, communication of control data (data of C-Plane) is performed via the radio base station, similarly to a conventional mobile communication system.
In general, a transmission acknowledgement signal (Ack/Nack signal), which indicates whether data transmitted from a transmission-side terminal has been able to be received, is transmitted from a reception-side terminal to the transmission-side terminal, so that retransmission control is performed.
Meanwhile, in the D2D communication, the radio base station is able to receive the user data communicated between the plurality of radio terminals. Consequently, in the D2D communication, it is possible to consider various methods as a method for the retransmission control.